The present invention is directed to a method for preparing materials for use in various microwave components and for radar absorbent coatings and the like; more particularly to a method for preparing dense spherical beads of hexagonal ferrites and their use in nonreciprocal microwave devices.
In the field of microwave communications, there are a variety of components generally referred to as nonreciprocal devices, such as circulators, isolators, tunable filters, and others. These passive, ideally lossless, devices are characterized by the ability to direct a microwave signal to various circuit elements depending on the direction from which the signal is coming. For example, when a transmitter and a receiver operate from a common antenna, a circulator is used to direct the output of the transmitter to the antenna while directing the incoming signal from the antenna to the receiver. Nonreciprocal devices generally rely on the phenomenon of gyromagnetic resonance in a ferrite body located within the device. The ferrite generally has an axis of symmetry (ellipsoid, needle, sphere, or disk) and is commonly a small sphere. A magnetizing field is applied to the ferrite body normal to the plane of the device, and the required magnetic field increases as the operating frequency of the device increases.
Circulators operating at microwave frequencies often employ a spherical bead of yttrium iron garnet (YIG) as the ferrite element. The YIG is grown as a single crystal (typically an inch or more in size) using a flux growth technique. The crystal is diced using a diamond saw into small cubes, which are tumbled with abrasive to round them into rough spheres. These are then polished between rotating platens (much like ball bearings). The polished bead is oriented and mounted on the end of a small polymer fiber. The degree of polish is important for minimizing losses during operation of the device. The practical upper frequency limit of a YIG oscillator is determined by the required DC magnetic field that must be applied. For example, at 96 GHz, the magnetic field would be about 30,000 gauss, which is clearly impractical to generate in, for example, a portable walkie-talkie or the like. As used herein, the term xe2x80x9cmicrowavexe2x80x9d includes frequencies from about 300 MHz to 300 GHz.
It is known in the art that a highly anisotropic material would be especially desirable for high frequency use because the internal anisotropy field of the material would reduce the necessary applied DC magnetic field. Hexagonal ferrites (BaFe12O19 and SrFe12O19) would be particularly useful in these applications if they could be formed into small highly polished spherical beads. Unfortunately, the mechanical properties of BaFe12O19 and SrFe12O19 are also very anisotropic (similar to mica), making it difficult or impossible to fabricate beads using the dicing and tumbling methods commonly employed for YIG beads. If, on the other hand, a method were available to make generally spherical BaFe12O19 and SrFe12O19 beads directly, these beads could then be polished without fracture because the polishing operation applies much lower stresses to the bead.
Accordingly, it is an object of the present invention to produce spherical beads of hexagonal ferrites for nonreciprocal microwave and mm-wave devices.
It is another object of the present invention to produce large numbers of substantially similar ferrite beads for incorporation into a polymer matrix or coating.
It is yet another object of the present invention to produce hexagonal ferrite components containing selected dopants to modify the magnetic properties.
It is a further object of the present invention to produce hexagonal ferrite components having a large grain size.
It is still yet a further object of the present invention to produce spherical beads of hexagonal ferrite components capable of being polished.
It is another object of the present invention to produce nonreciprocal microwave devices suitable for operation at mm-wave frequencies.
It is a further object of the present invention to produce hexagonal ferrite components by hydrothermal reaction of Fe2O3 with selected soluble salts, particularly alkaline earths.
Further and other objects of the present invention will become apparent from the description contained herein.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method for making a spherical bead of hexagonal ferrite comprising first forming a generally spherical porous bead of hydrous iron oxide by a sol-gel process. Then, reacting the bead with a soluble alkaline earth under hydrothermal conditions, then calcining the bead to form a sinterable, spherical bead of hexagonal ferrite and sintering the bead to a desired density and grain size.
In accordance with another aspect of the present invention, other objects are achieved by a microwave resonator comprising a sintered spherical bead of a hexagonal alkaline earth ferrite composition of the general formula AFe12O19 wherein A=Ba or Sr. The bead having been derived from a sol-gel process has a diameter less than about 1 mm and a grain size greater than about 0.1 xcexcm.
In accordance with yet another aspect of the present invention, other objects are achieved by a nonreciprocal microwave device having a resonator comprising a sintered spherical bead of a hexagonal alkaline earth ferrite composition of the general formula AFe12O19 wherein A=Ba or Sr. The bead having been derived from a sol-gel process has a diameter less than about 1 mm and a grain size greater than about 0.1 xcexcm.
In accordance with a further aspect of the present invention, other objects are achieved by a coating for selectively absorbing and reflecting microwaves comprising a polymeric matrix containing an inorganic dispersed phase. The dispersed phase contains substantially dense spherical sintered beads of a hexagonal alkaline earth ferrite composition of the general formula AFe12O19 wherein A=Ba or Sr. The beads having been derived from a sol-gel process followed by hydrothermal reaction have a final diameter less than 1 mm and a grain size greater than about 0.1 xcexcm.